1. Field of the Invention
The present invention relates to a heat transferable label and improved release composition therefor.
2. Description of the Prior Art
Prior art heat transferable labels for imprinting designs onto an article typically involve decorative laminates consisting of a paper base sheet or carrier web coated with a wax or polymeric release layer over which a design is imprinted in ink.
U.S. Pat. No. 3,616,015 (Kingston) is illustrative of the prior art. In U.S. Pat. No. 3,616,015 a label-carrying web, such as a paper sheet, includes a heat transferable label having a wax release layer affixed to a surface of the paper sheet and an ink design layer superimposed onto the wax release layer. In the heat transfer labeling process for imprinting designs onto articles, the label carrying web is subjected to heat and the laminate is pressed onto an article with the ink design layer making direct contact with the article. As the web or paper sheet is subjected to heat the wax layer begins to melt and allows the design layer to transfer to the arcticle. A portion of the wax release transfers to the article along with the design image. After transfer of the design to the article, the paper sheet is immediately removed leaving the design firmly affixed to the surface of the article with the wax layer exposed to the environment thereon. The wax layer thus serves two purposes in that provides release of the transferable label from the web upon application of heat to the web and also forms a clear protective layer over the transferred ink design. After transfer of the label to an article, the transferred wax release layer is typically subjected to post-flaming which produces an optically clear protective layer over the ink design and enhances the protective properties of the transferred wax release.
The additional step involving post-flaming is accomplished by subjecting the transferred wax layer to jets of high temperature gas either as direct gas flame or as hot air jets to produce wax surface temperatures of about 300.degree. to 400.degree. F. for a period of time sufficient to remelt the wax coating without substantially heating the article to which the label has been transferred. Upon cooling of the remelted wax coating through use of ambient or forced-cooled air, the cooled wax layer solidifies to form a clear, smooth protective coating over the ink design.
Although the heat transferable label disclosed in this reference may be utilized for decorating a wide variety of different articles, typically plastic bottles, there is a degree of hazing or "halo" noticable over the transferred label when the transfer is made onto clear plastic materials, despite use of post-flaming. The "halo" effect is caused by transfer of a portion of the wax release layer from the paper carrying sheet and onto the article along with the ink design layer. Although the transferred wax layer has the beneficial effect of providing a protective coating over the transferred ink design, the nature of the wax coating is such that it provides some halo around the outer borders of the transferred ink design layer. Although wax based release layers have produced optically clear protective layers over the ink design and provide a high degree of protection for the transferred ink design, they are nonetheless subject to scuffing and abrasion because of the inherent nature of the wax material.
U.S. Pat. No. 3,922,435 (Asnes) discloses a heat transferable label which is directed to replacing the wax based release layer with a non-wax resin thus avoiding the "halo" effect long associated with the use of wax based compositions. Asnes refers to this type of release layer as a dry release since it does not transfer to the article along with the ink design layer when heat is applied to the heat transferable laminate as the laminate is in contact with the article. In a preferred embodiment this reference discloses a dry release layer composed of a thermoset polymeric resin to impart to the layer in which it is present a softening temperature substantially greater than the temperature of the dry release transfer temperature, which is typically about 300.degree. to 450.degree. F., as disclosed in this reference. Preferred thermoset resins for the dry release layer disclosed in this reference are cross linked resins selected from the group consisting of acrylic resins, polyamide resins, polyester resins, vinyl resins and epoxy resins. The release layer, preferably composed of a thermoset resin, is overcoated with a lacquer layer which is in turn coated with the design print and then an adhesive overlayer. This reference teaches that the lacquer layer over the dry release layer also should have a softening temperature above the dry release heat transfer temperatures. (Col. 5, lines 58-60).
Although this reference is directed to use of thermoset resins for the dry release layer, the reference does state that certain thermoplastic resins, such as polypropylene can be used for the release layer so long as they have a softening temperature well above the temperature of the dry release transfer heat, that is, well above the range between 300.degree. to 450.degree. F. (See Col. 4, lines 49-53). In this connection this reference teaches that the use of polyethylene for the dry release composition has proved to be unsuitable. Asnes states that polyethylene "tend(s) to soften under heat transfer conditions, e.g., 300.degree. F.-450.degree. F., more usually 325.degree.-400.degree. F., required for commercially practical dry release heat transfer. This reduces the cohesion thereof and increases the adhesion thereof to the lacquer layer. As a result, during stripping, some of the polyethylene . . . is apt to be removed at least in some areas with the lacquer layer and design print, which remain adhered to the transferred surface by the heat activated adhesive, i.e., the cohesion of the resinous release layer is apt to be reduced at least in certain areas below the increased adhesion in those areas between the release layer and lacquer layer, and as a result, the break between the release and lacquer layers is not dependably and uniformly clean." (Col. 1, line 64 to Col. 2. line 10). This is a clear teaching against use of polyethylene as a composition for the dry release layer.
This well documented problem associated with the use of polyethylene for the release layer has long discouraged investigators in the art in attempting to employ polyethylene as a dry release composition for use in heat transferable labels wherein the required hot platen temperatures are in the conventional range between 300.degree. F. to 450.degree. F. It should be noted that this range of required platen temperature has in measure been set by the availability and use of conventional heat activatable adhesives which are used to overcoat the ink design layer or included in the ink design layer. Conventional heat activatable adhesives, which have been found to be suitable for use in this technology have required a platen temperature heat source in the range between about 300.degree. F. to 450.degree. F. The teaching of this reference is that when subjected to the required heat between 300.degree. F. to 450.degree. F. to effect label transfer, the polyethylene becomes instantly more adhesive and less cohesive, thus preventing attainment of a uniformly clean release. This is a clear teaching against the use of polyethylene for the release composition. These teachings are representative of this long standing problem associated with the use of polyethylene as the dry release composition in applicant's art. Such teachings discourage the use of polyethylene as a dry release composition for commercially acceptable heat transfer labels.
Accordingly, it is an object of the present invention to provide and improve nonwax based release system for heat transferable laminates which permits transfer of an ink design image from a carrier web to an article, in particular to a plastic article.
It is an important object to provide a release system which provides a protective coating over the transfer ink design image such that the transferred image shows improved resistance to abrasion and scuffing while maintaining a high degree of optical clarity.
It is a further object to provide an improved release system and heat transferable laminate for use in transfer of a design image from a carrier web to a plastic article wherein the problem of wax "halo" around the transferred image has been eliminated. A related object is to provide an improved release system for heat transferable laminates which provides a protective coating for the transferred image which is also resistant to common solvents.